Migrate native builds to Dart code assets

[peerchemist]

• Replace frosty_flutter and Cargokit with a Dart code-assets build hook.
• Build and bundle the underlying Rust library directly from the frosty package.
• Consolidate the Flutter example and update CI for all platforms.
• Require Dart 3.13 and Coinlib 6.
• Remove obsolete native build scripts.